Attempts were made to measure tryptophan hydroxylase activity in blood platelets. Under conditions promoting maximal tryptophan hydroxylation in other tissue extracts (e.g., pineal gland or rat brain) and cultures (pineal), no tryptophan hydroxylase activity was found in platelets isolated from human or rat blood. These results suggest that the serotonin found in platelets is largely accumulated via an active uptake mechanism and cannot be attributed to the hydroxylation of tryptophan to serotonin within this plasma organelle. BIBLIOGRAPHIC REFERENCE: Morrissey, J.J., Walker, M.N. and Lovenberg, W.: The absence of tryptophan hydroxylase activity in blood platelets. Proc. Soc. Exp. Biology and Med. 154: 496-499, 1977.